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AD515A Datasheet(PDF) 3 Page - Analog Devices

Part No. AD515A
Description  Monolithic Precision, Low Power FET-Input Electrometer Op Amp
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD515A Datasheet(HTML) 3 Page - Analog Devices

   
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–3–
REV. A
AD515A
LAYOUT AND CONNECTIONS CONSIDERATIONS
The design of very high impedance measurement systems in-
troduces a new level of problems associated with the reduction
of leakage paths and noise pickup.
1. A primary consideration in high impedance system designs is
to attempt to place the measuring device as near to the signal
source as possible. This will minimize current leakage paths,
noise pickup and capacitive loading. The AD515A, with its
combination of low offset voltage (normally eliminating the
need for trimming), low quiescent current (minimal source
heating, possible battery operation), internal compensation
and small physical size lends itself to installation at the signal
source or inside a probe. As a result of the high load capaci-
tance rating, the AD515A can comfortably drive a long
signal cable.
2. The use of guarding techniques is essential to realizing the
capability of the ultralow input currents of the AD515A.
Guarding is achieved by applying a low impedance bootstrap
potential to the outside of the insulation material surround-
ing the high impedance signal line. This bootstrap potential
is held at the same level as that of the high impedance line;
therefore, there is no voltage drop across the insulation and,
hence, no leakage. The guard will also act as a shield to
reduce noise pickup and serves an additional function of
reducing the effective capacitance to the input line. The case
of the AD515A is brought out separately to Pin 8 so it can
also be connected to the guard potential. This technique
virtually eliminates potential leakage paths across the package
insulation, provides a noise shield for the sensitive circuitry
and reduces common-mode input capacitance to about 0.8
pF. Figure 1 shows a proper printed circuit board layout for
input guarding and connecting the case guard. Figures 2 and
3 show guarding connections for typical inverting and
noninverting applications. If Pin 8 is not used for guarding, it
should be connected to ground or a power supply to reduce
noise.
Figure 1. Board Layout for Guarding Inputs with Guarded
TO-99 Package
3. Printed circuit board layout and construction is critical for
achieving the ultimate in low leakage performance that the
AD515A can deliver. The best performance will be realized
by using a teflon IC socket for the AD515A; at a minimum a
teflon standoff should be used for the high impedance lead.
If this is not feasible, the input guarding scheme shown in
Figure 1 will minimize leakage as much as possible; the
guard ring should be applied to both sides of the board. The
guard ring is connected to a low impedance potential at the
same level as the inputs. High impedance signal lines should
not be extended for any unnecessary length on a printed
circuit; to minimize noise and leakage, they must be carried
in rigid, shielded cables.
4. Another important concern for achieving and maintaining
low leakage currents is complete cleanliness of circuit boards
and components. Completed assemblies should be washed
thoroughly in a low residue solvent such as TMC Freon or
high purity methanol, followed by a rinse with deionized
water and nitrogen drying. If service is anticipated in a high
contaminant or high humidity environment, a high dielectric
conformal coating is recommended. All insulation materials
except Kel-F or teflon will show rapid degradation of surface
leakage at high humidities.
Figure 2. Picoampere Current-to-Voltage Converter
Inverting Configuration
Figure 3. Very High Impedance Noninverting Amplifier
INPUT PROTECTION
The AD515A is guaranteed for a maximum safe input potential
equal to the power supply potential.
Many instrumentation situations, such as flame detectors in gas
chromatographs, involve measurement of low level currents
from high voltage sources. In such applications, a sensor fault
condition may apply a very high potential to the input of the
current-to-voltage converting amplifier. This possibility necessi-
tates some form of input protection. Many electrometer type
devices, especially CMOS designs, can require elaborate Zener
protection schemes that often compromise overall performance.
The AD515A requires input protection only if the source is not
current limited and, as such, is similar to many JFET-input
designs. The failure mode would be overheating from excess
current rather than voltage breakdown. If the source is not
current limited, all that is required is a resistor in series with the
affected input terminal so that the maximum overload current is
0.1 mA (for example, 1 M
Ω for a 100 V overload). This simple
scheme will cause no significant reduction in performance and
give complete overload protection. Figures 2 and 3 show proper
connections.


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